[1] Linkoping Univ Hosp, Dept Pediat, S-58185 Linkoping, Sweden
[2] Karolinska Univ Hosp & Inst, Dept Med, Clin Epidemiol Unit, Stockholm, Sweden
[3] Univ Calif Berkeley, Sch Publ Hlth, Berkeley, CA 94720 USA
[4] Karolinska Inst, Dept Med Epidemiol & Biostat, Stockholm, Sweden
[5] Univ Hosp, Dept Pediat, Orebro, Sweden
[6] Univ Nottingham, Sch Med, Div Epidemiol & Publ Hlth, Nottingham, England